I would think the poly count of the entire scene is more important than one object. I think the general idea is to get as few polys as possible while keeping an acceptable level of detail. I know that's vague, but there really is no good answer in terms of number of polys since it depends on so many things.

From what I understand, one process for making game models is to make a high poly version, then retopo to make a low poly one. Then you map the details of the high poly to the low poly in the form of normals, bump maps, spec maps, image textures, etc. Your model will be the general shape of the object and the details are stored in the images for your textures.

In the end though, all you can really do is test the game. If it crashes, tweak your model/textures/lighting. The more calculations your game as to make while you are playing, the slower it will be. As I understand it, cutting calculations means storing more details into normal/bump/spec/lighting/etc maps. That said, I'm no expert, so I could be overgeneralizing quite a bit.